Thanks to an anti-Google smear campaign ordered by Facebook and carried out by a PR agency, the relationship between Facebook and Google is unquestionably broken beyond repair. And that’s bad news for users of both services.

The dirty deed sinks the Google-Facebook rivalry to a new low, while highlighting how the giants of search and social networking are increasingly at odds. According to The Daily Beast, two representatives of PR firm Burson-Marsteller tried to solicit an independent blogger and USA Today to attack Google’s privacy approach, particularly as it applies to social search results. When confronted with evidence, Facebook confirmed that it hired the agency to carry out the campaign.

As with previous scuffles between the two companies, this incident was all about user data, and who gets to control it. Facebook doesn’t like how Google scrapes public Facebook data for its own social search results, and therefore claimed that the data was being used improperly. Google is looking into that allegation.

There are legitimate issues here: Google does, in fact, look at public Facebook information as part of Social Search, a type of search result that’s based on links shared by people you know. If you use Google products, you can look at how Google gathers information from your contacts. Whether this is a violation of privacy should be up for debate.

But Facebook’s use of a covert smear campaign only gets in the way of clear-headed discussion, and that’s a shame at a time when both companies are under scrutiny. I envision a future in which both companies try to spin themselves as superior protectors of users’ data, when they should really concentrate on being honest and open with users.

Meanwhile, it’s unlikely that Facebook and Google will ever go back to sharing their data for the benefit of users. It’s too bad, because Facebook could use Gmail users’ contact lists to suggest new friends, while Google could use Facebook’s vast database of “Likes” to create better search results. The companies were already acting like children before, with back-and-forth bickering over other peoples’ data, and now they’ve grown up to be mortal enemies.